The present invention relates to a method of supplying a plasma torch with a gas, mixed gas or gas mixture, in which the volume flow of the gas, mixed gas or gas mixture is controlled. In addition, the present invention relates to an arrangement for supplying a plasma torch with a gas or mixed gas or gas mixture, with an arrangement for delivering the gas or mixed gas or gas mixture to the plasma torch and a volume flow control for controlling the volume flow of the gas or mixed gas or gas mixture.
Various gases are used as plasma gases, such as monatomic argon and/or the diatomic gases hydrogen, nitrogen, oxygen or air. These gases are ionized and dissociated by the energy of the plasma arc. A plasma mixed gas is a plasma gas that has already been premixed by the supplier, whereas a plasma gas mixture is a plasma gas that is mixed on the spot.
As a rule, the plasma in a plasma torch is constricted by a water-cooled nozzle. In this way, energy densities of up to 2×106 W/cm2 can be achieved. In the plasma arc of a plasma torch, temperatures of up to 30,000° C. arise, which, in combination with the high flow rate of the plasma gas, result in very high cutting speeds in all electrically conductive materials.
For a plasma cutting process, a pilot arc is first ignited between the nozzle and the cathode of the plasma torch with a high voltage. This low-energy pilot arc prepares the path between the plasma cutting torch and the work piece using partial ionisation. When the pilot arc touches the work piece, a cutting arc forms.
Plasma cutting is an established method of cutting electrically conductive materials. Depending on the cutting task, various gases and gas mixtures are used. Conventional gases and gas mixtures are, for example, air, oxygen, nitrogen and their gas mixtures, and also argon/hydrogen/nitrogen mixtures.
Unalloyed steels are normally cut with air or oxygen. Alloyed steels and non-ferrous metals are preferably cut with special argon/hydrogen, nitrogen/hydrogen or argon/hydrogen/nitrogen mixtures. In order to improve the quality of a cut, an additional secondary gas can also be employed, which also flows around the plasma jet. The function of the additional secondary gas is to protect the nozzle of the plasma torch against material from the work piece splashing back when piercing the work piece, thus protecting against damage. The additional secondary gas also influences the molten metal during cutting in such a way that the cut produced is free of dross and acts as a shield gas to protect the surface of the cut, which is still hot after it has been cut, against oxidation.
The plasma and secondary gases, mixed gases and gas mixtures are delivered to the plasma cutting torches via lines and solenoid valves. Gases are usually metered by setting or adjusting pressure. Pressure can be controlled either mechanically via pressure reducers, or electronically via pressure control valves. The use of electronic pressure regulators is common, especially in automated systems, in which a wide range of plasma cutting parameters, such as cutting current, cutting voltage, gas pressure, cutting rate, the thickness of the material and plasma cutting torch distance are stored in databases in order to achieve the greatest possible reproducibility of cutting results.
German document DE 195 36 150 C2, for example, describes a means and method for gas control in a plasma torch, in which gas flow is set by an arrangement consisting of a proportional valve, a pressure sensor and a shield in a plasma torch.
In European Patent Specification EP 0 697 935 B1, gas is metered by means of changeable needle valves. The cross-section of the needle valves, in combination with the pressure set, determines the amount of gas. Volume flow can be indicated in the process by means of variable-area flow meters.
Particular gas mixtures needed for processing alloyed steels and non-ferrous metals cannot however be reproducibly generated by means of pressure control. Attempts have therefore been made to reduce this disadvantage by using auxiliary devices. East German document DD 54437, for example, describes the use of a mixing chamber with pressure shields. However, this does not solve the problem since the resulting mixing ratio is very restricted.
It is particularly difficult to mix gases of different densities and in widely varying mixing ratios. For example, even the use of various known mixing devices, including T-fittings, injectors, labyrinth arrangements and arrangements of nozzles, as are described in East German Document DD 132247, cannot produce the varying optimum mixing ratios required.
It is possible to perform gas metering by means of pure volume flow control. This method can reproducibly create defined gas mixtures.
U.S. Pat. No. 6,248,972 B1 discloses a method and arrangement for reducing electrode and nozzle wear in oxygen plasma cutting by using a mixture of oxygen and nitrogen instead of pure oxygen. In the method, a constant volume flow of the individual plasma gases is produced by means of an arrangement consisting of needle valves and differential pressure gauges, such that the differential pressure upstream and downstream of the differential pressure gauges is kept constant by means of the needle valves upstream. Between the controlled member and the plasma torch there are pressure-reducing valves, which limit the maximum supply pressure to the plasma torch.
German utility model DE 201 21 641.8 U1 describes a method of supplying a plasma torch with a gas, mixed gas, or gas mixture, in which the volume flow of the gas, mixed gas, or gas mixture is controlled, using an arrangement for supplying a plasma torch with a gas or mixed gas or gas mixture with a means for delivering the gas or mixed gas or gas mixture to the plasma torch and a volume flow control means for controlling the volume flow of the gas or mixed gas or gas mixture.
However, even if a volume flow control is able, depending on the measuring method used, to generate a more or less constant volume flow of a gas or gases and can consistently and accurately reproduce such a gas mixture, the quality of a resulting cut is inadequate. This is especially true at the start of cutting a given material. Such inadequate quality cuts can take various forms. A lack of or delayed transfer of a pilot arc can lead to unreliable piercing of a work piece to be cut. In some cases, unreliable through cuts can occur and material can be left behind on the work piece. The formation of dross can occur and slag can form on the underside of the work piece. A major deviation in the desired angle can also lead to exceeding the tolerances of a desired right angle or slope of the work piece.